the popup is opening before the content is loaded so it is not the correct size upon open. Can you call the popup open after loading the data within the post success function:
function DisplayVisualCheckPopUP(unitid) {
$.post("visualCheck.php", "unitid="+unitid, function(data){
$("#frmVisualUnit").html(data);
$("#frmVisualUnit").trigger('create');
$("#popupVisualUnit").popup("open");
});
};
In this way the popup will already be populated when it is displayed.